Re: [PATCH RFC 14/20] proc: store cookie in private data

[Christian Brauner <brauner@xxxxxxxxxx>]

On Fri, Aug 30, 2024 at 03:04:55PM GMT, Christian Brauner wrote:
> Store the cookie to detect concurrent seeks on directories in
> file->private_data.
> 
> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> ---
>  fs/proc/base.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 72a1acd03675..8a8aab6b9801 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3870,12 +3870,12 @@ static int proc_task_readdir(struct file *file, struct dir_context *ctx)
>  	if (!dir_emit_dots(file, ctx))
>  		return 0;
>  
> -	/* f_version caches the tgid value that the last readdir call couldn't
> -	 * return. lseek aka telldir automagically resets f_version to 0.
> +	/* We cache the tgid value that the last readdir call couldn't
> +	 * return and lseek resets it to 0.
>  	 */
>  	ns = proc_pid_ns(inode->i_sb);
> -	tid = (int)file->f_version;
> -	file->f_version = 0;
> +	tid = (int)(intptr_t)file->private_data;
> +	file->private_data = NULL;
>  	for (task = first_tid(proc_pid(inode), tid, ctx->pos - 2, ns);
>  	     task;
>  	     task = next_tid(task), ctx->pos++) {
> @@ -3890,7 +3890,7 @@ static int proc_task_readdir(struct file *file, struct dir_context *ctx)
>  				proc_task_instantiate, task, NULL)) {
>  			/* returning this tgid failed, save it as the first
>  			 * pid for the next readir call */
> -			file->f_version = (u64)tid;
> +			file->private_data = (void *)(intptr_t)tid;
>  			put_task_struct(task);
>  			break;
>  		}
> @@ -3915,6 +3915,12 @@ static int proc_task_getattr(struct mnt_idmap *idmap,
>  	return 0;
>  }
>  
> +static loff_t proc_dir_llseek(struct file *file, loff_t offset, int whence)
> +{
> +	return generic_llseek_cookie(file, offset, whence,
> +				     (u64 *)(uintptr_t)&file->private_data);

Btw, this is fixed in-tree (I did send out an unfixed version):

static loff_t proc_dir_llseek(struct file *file, loff_t offset, int whence)
{
	u64 cookie = 1;
	loff_t off;

	off = generic_llseek_cookie(file, offset, whence, &cookie);
	if (!cookie)
		file->private_data = NULL; /* serialized by f_pos_lock */
	return off;
}